Oilpan: Move LifecycleNotifier's hierarchy to Oilpan's heap

Source/platform/Supplementable.h

Index: Source/platform/Supplementable.h
diff --git a/Source/platform/Supplementable.h b/Source/platform/Supplementable.h
index ae8f8fc5a82031e08897d15fb0ef229662dabbec..41d8cea7bf9e7a50e1932bc657ef7367b67acbf8 100644
--- a/Source/platform/Supplementable.h
+++ b/Source/platform/Supplementable.h
@@ -152,8 +152,10 @@ public:
 template<typename T, bool isGarbageCollected>
 class SupplementableTracing;
 
+// Using a virtual inheritance to resolve a diamond inheritance
+// in ExecutionContext.
 template<typename T>
-class SupplementableTracing<T, true> : public GarbageCollectedMixin {
+class SupplementableTracing<T, true> : public virtual GarbageCollectedMixin {
 public:
     virtual void trace(Visitor* visitor)
     {